Replies

1. 
   Devin Ratliff|May 19, 2011 05:20pm|#1

   Minn D,

   It sounds like you're talking about a frost-protected shallow foundation. The IRC recognizes this type of foundation, and has criteria about how thick the foam has to be and how far it has to extend from the foundation wall, but it does not sound like it is at all applicable for what you are describing. An FPSF relies on a continuously heated structure (not allowed for unheated structures) to raise the frost level adjeacent to the building, so that you don't have to dig as deep. But it sounds like what you're describing is a secondary structure outside of your buidling's foundation. Just putting down foam doesn't lower the frost depth, which is the reason for the 42". Maybe if you skirted our from your building foundation beyond your new foundations - but it's a big stretch.

   I think you're going to need that posthole digger.

2. GBA Editor
   Martin Holladay|May 19, 2011 05:48pm|#2

   Drat,
   You're wrong when you write, "An FPSF relies on a continuously heated structure (not allowed for unheated structures) to raise the frost level adjacent to the building."

   For more information on these foundations, see my article in Fine Homebuilding:
   http://www.finehomebuilding.com/design/departments/energy-smart-details/frost-protected-shallow-foundations.aspx?ac=fp

   In that article, I noted, "Fortunately, these shallow foundations don’t depend on leaking building heat to keep the soil warm. Instead, horizontal wing insulation extending from the bottom edge of the slab helps to retain the natural warmth of the earth."

   Also, see this document:Revised Builder’s Guide to Frost Protected Shallow Foundations.
   Design details for unheated buildings begin on page 17 of that document.

3. 
   Scorched Earth, 3B|May 19, 2011 07:02pm|#3

   Thank you. The Revised FPSF Guide is the document I was thinking of.

   So, with an incomplete understanding of the issue, I am asking if it's true that I may indeed set an inch of EPS under my footers for this trellis/pergola/whatever, and dig them to a depth of only 16"+ if I'm in a 3,000 HDD location. Page 28:

   "Finally, because of the insulation, the frost line will not penetrate as deeply into the soil around the foundation."

4. 
   Scorched Earth, 3B|May 19, 2011 07:03pm|#4

   By the way, the first 12" or so down are the toughest anyway, so it's not going to kill me either way. But I'd like to know.

5. 
   Devin Ratliff|May 20, 2011 06:58am|#5

   Thanks for the clarification and links Martin; I stand corrected (once again, I wish that I'd found GBA sooner!)

   In this case though, I think the answer goes from 'not allowed' to 'not practical'. Check out those horizontal insulation values for unheated structures in the buider's guide: with an air freezing index of 2000-3000, you're looking at an unmodified 63-79" of horizontal insulation! Unless you can harness the heat of your house (and even if you can, you're looking at 24" horizontal as a corner condition), it sounds way easier to dig down to frost.

   Minneapolis, I think what you're suggesting is simply laying a board of insulation under your post foundation, which won't work. An FPSF relies on a skirt of horizontal insulation around the structure - either to retain the building's heat (less of a skirt required because there's more heat there) or the earth's heat (more of a skirt required because there's less heat there). If the first 12" of digging is the hardest, then an SPSF (shallow but wide) is going to be a lot more work than going down to frost (narrow, but deep).

6. 
   J Chesnut|May 21, 2011 10:48am|#6

   Minne D,

   I am asking if it's true that I may indeed set an inch of EPS under my footers for this trellis/pergola/whatever, and dig them to a depth of only 16"+ if I'm in a 3,000 HDD location.

   Minneapolis is a 8000 HDD climate not 3000.
   Maybe it was a typo in post #3, if not it might change your depth determination.

7. 
   Scorched Earth, 3B|May 21, 2011 02:23pm|#7

   Haha, thanks. In any case if I go to 42" uninsulated, I'm set.